Bret Clanton might not belong to the most obvious group of opponents to the proposed Keystone XL pipeline. But when a survey crew from TransCanada arrived on his property eight years ago, the rancher and registered Republican – worried they were cattle thieves – says he called the sheriff’s department, and marched out to confront them.

He says the encounter changed his life, and set up a battle that would come to dominate his existence.

On the outer perimeter of the Clanton ranch, where three large sandstone rock formations stand over the otherwise empty horizon, TransCanada later told him it planned to dig up three miles of his land and lay a section of the Keystone XL pipeline. TransCanada also hopes to bulldoze along another two and half miles of his pastures to make way for an access road.

“I’ve lived here all my life and this ground is pretty much as God, or whoever, made it, and I just want it stay that way,” he says.

Clanton fought from the beginning and lobbied the state government for several years. But he was made aware almost instantly it was likely to be a losing fight. “From the very first meeting [with TransCanada] I was informed they would have power of condemnation,” he says.

The bleak assessment was correct. As soon as the XL route had received the necessary state approval in South Dakota back in 2010, TransCanada was essentially able to seize control of any private land it needed, in return for a fee, through the power of eminent domain.

A principle protected by the fifth amendment to the US constitution, eminent domain permits the seizure of private land for “public use” in exchange for “just compensation”. It was used by government at the turn of the 19th century, when Clanton’s great grandfather arrived in South Dakota on horseback, driving cattle from the south, to reclaim land for public infrastructure and to preserve historic sites.

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But emiment domain has since morphed to empower various gas and oil pipeline companies to construct across the US, buoyed by a 2005 supreme court ruling that found private entities providing “economic development” were essentially providing a public service.

Advocates estimate that dozens of landowners in South Dakota and Montana were essentially forced to settle with TransCanada to avoid eminent domain enactments during the Obama administration.

Close to the north-east border with Montana, Clanton’s remote ranch, outside the small town of Buffalo, is one of the first properties the XL will cross in South Dakota. It marks the Guardian’s first stop-off in the state during a trip that traces the proposed pipeline’s entire length through the US to examine the battles that have reignited after the Trump administration resurrected the project.

It is not just environmentalists and Native Americans lined up against the construction. Registered Republican ranchers like Clanton are also among the opposition, pitting them against the leaders of their own party who have long supported the pipeline.

Every Republican candidate in the 2016 presidential race supported the project. Donald Trump, himself a frequent user of eminent domain in private business, described the power as “wonderful” during the campaign.

Clanton will not say how he voted last year and does not want to engage with the obvious contradictions between his preferred party and his current situation.

“If Hillary Clinton had been elected, she would’ve done the same thing. It was inevitable,” he says. (During the election, Clinton pledged to oppose the project but had expressed tentative support for it during her tenure as secretary of state.)

Clanton is clinging to the hope that if oil prices continue to fall, the construction may no longer be economically viable. But he expresses a certain resignation already. Perhaps the only landowner still holding out TransCanada in South Dakota lives 350 miles south-east of Clanton’s ranch.

South Dakota state senator Troy Heinert tends to his rodeo horses on the Rosebud Indian Reservation. Photograph: Laurence Mathieu-Leger for the Guardian

John Harter’s ancestors have ranched cattle close to the south-western border with Nebraska since 1939. The XL is scheduled to bisect a 280-acre grazing pasture that has been in the family since then, and the 54-year-old is preparing to battle until the bitter end.

Guided by Christian faith and the values he learned through Shotokan karate – Harter is a level-five black belt – he says: “I don’t believe in being bullied, and I won’t be bullied.”

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TransCanada eventually took Harter to eminent domain, he claims. But he is now arguing that the easement is void because of what he describes as a breach of contract.

“I have told TransCanada that they do not have a legal easement to my property, and if they entered my property, that I would remove them, and if necessary with force,” Harter says.

TransCanada says it has completed easements with every landowner on the XL route within Montana and South Dakota. “We have been working with Mr Harter for several years and we are committed to working with him in the future,” said TransCanada spokesman Terry Cunha.

Harter drives out to the pasture they plan to bulldoze, his navy blue 1978 Ford F-150 bouncing on tufts of windswept wild grass, and reveals he, too, is a registered Republican. The contradiction remains striking, and Harter is clearly discomforted by it.

“Being a conservative is one thing,” he says about those in his party who support the project. “Being a blind conservative is another.”

But it is not only these ranchers’ right to their land that is under threat in South Dakota. It is their right to protest, too.

•••

The 150 miles from Clanton’s pastures to the town of Eagle Butte, in the heart of the Cheyenne River Reservation, is bathed in intense sunlight that colours the distant mountain ranges in a wash of blue. Here, small casinos litter the roadside, and tumbleweed rolls across the streets.

The XL is is set to pass just outside the reservation, and will cross underneath the Cheyenne river, a major tributary of the Missouri river and the place the reservation’s Sioux tribe get their water from.

Last month, South Dakota’s Republican governor Dennis Daugaard, a longtime supporter of the XL, signed Senate Bill 176. The new law allows local officials to prohibit groups larger than 20 from congregating on state land, and could criminalize those who attempt to protest on state highways. (The Montana legislature is considering similar legislation.) It was pushed through, without consultation with tribal leaders in the state, as an emergency measure, a follow-on response to the direct action protests at Standing Rock in North Dakota.

Many Native Americans here view the new law as a further assault on their rights after centuries of disenfranchisement. Hundreds are already veterans of Standing Rock, which borders this reservation to the north.

The council chairman Harold Frazier was present throughout these protests, watching as his contemporaries were teargassed and shot with rubber bullets by police.

“It is something I will never forget,” he says, later branding the new law unconstitutional and suggesting it will be challenged in court. “It was so awesome to see the strength of our people.”

Protest camps, set up to oppose the XL, are already starting to populate at Eagle Butte and other locations throughout the reserve, but leaders won’t discuss their plans in detail. As it is on the Fort Peck reservation in Montana, people here are furious about the perceived lack of consultation with TransCanada and government officials. The renewed fight against the XL is coupled with a desire to galvanize the community here against the many struggles they face.

Almost 40% of residents in Eagle Butte live below the poverty line, and in the hours before the Guardian arrived, the community held a funeral for a nine-year-old girl who killed herself days before. There had been 20 suicide attempts by young girls on the reserve in the past two weeks, a tribal spokesman said.

Ninety miles south, in the state capital of Pierre – a slight detour from the XL’s proposed route – the governor was not available to be interviewed about the new law. Instead, the administration put up a member of Daugaard’s cabinet, who is adamant the legislation does not discriminate against Native American protesters.

I certainly don’t think it is going to make any great economic impact on South Dakota or on the native tribes

Steve Emery, Republican secretary for tribal relations

Steve Emery, the Republican secretary for tribal relations and himself a member of the Rosebud Sioux Tribe, is, of course, expected to hold the governor’s line on the issue. But during an interview at his secretarial office, he made an honest admission that is likely to frustrate his boss by undermining the administration’s primary justification for supporting the XL in the first place.

“I certainly don’t think it [the XL pipeline] is going to make any great economic impact on South Dakota or on the native tribes that share our borders,” he says.

Then what is it that the tribes in this state do not understand about the pipeline that explains the Daugaard administration’s support for it?

He pauses, takes a sip of water, and, after an awkward silence, responds: “I am not really sure how to answer that, because I am sure they probably know more about the pipeline than I do.”

•••

Emery’s Rosebud reservation is the last large population concentration the XL will affect in South Dakota. The pipeline is set to cross just north of the reserve, which sits at the very top of the Ogallala aquifer. This groundwater source is one of the world’s largest aquifers, covering around 174,000 sq miles and crossing underneath eight states, supplying drinking water to nearly 2 million people.

Even a small undetected leak in this area, according to the independent analysis conducted by engineering professor John Stansbury, could contaminate surrounding water sources with intolerable benzene levels “posing serious health threats to anyone [in the vicinity] using the underlying Ogallala aquifer for drinking water or agriculture”.

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Troy Heinert says he is one of those in the potential crosshairs. The aquifer supplies water to his herd of rodeo horses that perform at events around the country. Heinert is also Emery’s cousin and a Democratic state senator, one of only a handful of Native Americans in the Republican-dominated state legislature. He was one of the few to vote against Senate Bill 176, which he brands “dangerous”. Activists on this reserve are planning to resist construction if a last ditched state court appeal against the permit in South Dakota fails.

Heinert empathises with his cousin’s inability to articulate a single reason Native Americans should warm to the project.

“South Dakota has very clean water,” he says, as his herd of horses gallop over the pasture to come in to drink. “We have over 300,000 people who receive water from rural water systems that could be affected by this. We have no plan if something were to happen.”